The field of the invention is external beam radiation therapy systems. More particularly, the invention relates to collimators for use in radiation treatment systems, including stereotactic radiosurgery (“SRS”) systems.
SRS is a single fraction radiation therapy technique that uses x-ray radiation in order to deliver a high dose to a localized treatment region. For a standard linear accelerator radiosurgery treatment based on CT localization, it has been shown that the irradiation of a target has a positional accuracy of 0.8 mm in any direction. This offers an advantage as opposed to other radiotherapy techniques in that SRS is able to provide a large dose to a small target region while sparing much of the normal tissue surrounding the tumor. The precise localization of radiation has allowed for SRS to excel in the treatment of small tumors, particularly within the brain, such as metastases or gliomas. While the primary uses are in the brain, the application of SRS to treatments in other regions of the body, such as the liver, lungs or spinal cord, has expanded over recent years as well.
Stereotactic radiosurgery does not come without its set of drawbacks, however. A large limitation in SRS is the extent to which it can be applied to treatments. Largely, SRS can only be used for small, well defined tumors. As was mentioned previously, the treatment is highly localized in its position, and if the tumor position and extent cannot be accurately determined, the accuracy of SRS would be negated. An additional drawback associated with SRS is that the length of a single treatment may be relatively long (up to an hour for a 4 mm single isocenter treatment, and several hours for multiple isocenter treatments). This is largely due to the use of multiple isocenters for each treatment, as well as the fact that the beam is subject to a large amount of collimation in order to achieve the high accuracy associated with SRS. As the beam is being collimated down to only a few millimeters, the bulk of the radiation output from the linear accelerator is simply scatted within the collimator until it runs out of energy. Subsequently this leads to a low output reaching the patient, and thus a low efficiency.
Thus, there remains a need to enable SRS treatments in less time than is currently achievable without reducing the accuracy of the SRS treatment system or unduly limiting the therapeutic effect of the radiation dose imparted to the patient.